As a fixing roll for electrophotography, there has been used a roll comprising a cylindrical core made of aluminum, iron or stainless steel coated with an easily releasable material, for example, a fluoro resin such as a polytetrafluoroethylene (hereinafter referred to as PTFE resin), a copolymer of tetrafluoroethylene and perfluoroalkyl perfluorovinyl ether (hereinafter referred to as PFA resin).
Such a fixing roll is opposed under pressure to a press roll coated with a silicone rubber or fluoro rubber to constitute a fixing device. Copy paper is passed between the heated fixing roll and the press roll to fuse powdery toner images formed thereon by means of heat and pressure and fix them onto the copy paper. An image formed on the paper by toner particles is pressed under heat so as to be fused and fixed on the paper.
One method employed for manufacturing a fixing roll having a coating layer of the fluoro resin is a method of coating a fluoro resin powder or fluoro resin dispersed liquid onto the surface of the core and then sintering it at a temperature of 360.degree. to 400.degree. C. to effect fusion deposition thereof on the surface. However, the thickness of the coating layer is not always uniform because of unevenness left after the coating. Since the nonuniformity in the thickness of the fluoro resin layer changes the press-contact force upon toner fixation, deterioration of the quality of the fixed image results. Therefore, the fluoro resin layer after sintering has conventionally been ground into a predetermined thickness by using sandpaper or a grinding stone, also improving the smoothness of the surface.
However, such a surface grinding involves the problems of causing deep scratches and of taking many working steps. Accordingly, there has been proposed a method of smoothing the surface by a press roll instead of grinding it (refer to Japanese Patent Laid-Open Sho 57-43892).
The smoothing of the fixing roll surface by the above-described conventional method is effective for improving the quality of the fixed image. However, it is not always effective for the improvement of an offset phenomenon, that is, a phenomenon that toner particles fused upon fixing of the toner image are deposited onto the fixing roll and the deposited toner particles are transferred to the subsequent copy paper as contaminant.
The offset phenomenon is generally classified into a so-called high temperature offset and a so-called low temperature offset. The high temperature offset is caused by the deposition of the fused toner on the fixing roll, while the low temperature offset is caused by the deposition of unfused toner particles on the fixing roll, which are determined depending on the distribution of the surface temperature of the fixing roll, drop in temperature upon paper passage, toner characteristics, etc. Accordingly, it is required for the fixing roll that the temperature for causing the high temperature offset is higher and the temperature for causing the low temperature offset is lower. However, the fixing roll obtained by the above-mentioned conventional method involves a problem that the temperature ranges for causing the high temperature offset and for causing the low temperature offset are close to each other and thus, the non-offset temperature range is narrow.